Cross-links From Abasic Sites in Duplex DNA. A significant ongoing endeavor in cancer research and environmental toxicology involves the identification of important DNA-damage lesions and characterization of their ability to induce cell death or cancer-causing mutations. The proposed work will characterize a novel family of DNA lesions that are derived from apurinic/abasic (AP) sites in duplex DNA. AP sites are generated by a wide variety of processes and may be the most common type of damage sustained by cellular DNA. This proposal builds upon our recent observations that AP sites can generate interstrand DNA-DNA cross-links via reactions with nucleobases on the opposing strand of the double helix. It is striking that AP sites can generate interstrand cross-links, which generally ar thought to be the most deleterious of all DNA lesions. Cross-links present an exceptional challenge to the DNA-repair machinery in human cells. The repair of cross-links may be error- prone and the resulting mutations in the genetic code could contribute to the etiology of both spontaneous and chemical-induced cancers. Left unrepaired, cross-links may block DNA transcription and replication. The exact nature of the cellular response(s) to these cross-links may be influenced by an individual's genetic makeup, especially with regard to defects or polymorphisms in their DNA damage response and repair machinery. The proposed work is significant because it will characterize how cross-links contribute to the propensity for endogenous or chemically-induced abasic sites to cause mutagenesis, cancer, cell dysfunction, senescence, and aging in humans. Our studies represent the first efforts to characterize the formation and biological consequences of these recently discovered DNA lesions. The proposed work will: (Aims 1 and 2) characterize the formation and chemical structures of two different AP-derived cross-links in duplex DNA, (Aim 3) use mass spectrometric methods to quantitatively measure the formation and repair of AP-derived cross-links in human cells exposed to agents that induce AP sites, including radiolysis and clinically-used alkylating agents and, (Aim 4) assess transcriptional bypass, replication, and repair of the dG-AP cross- link in human cells using a shuttle-vector methodology.